This invention relates to a method of producing graphite fluoride in the form of ultrafine particles excellent in dispersibility by direct fluorination of carbon black.
Graphite fluoride is a common name of polycarbon fluorides represented by (CF.sub.x).sub.n, wherein x is up to about 1.3. At present, most of graphite fluorides on the market are either (CF).sub.n or (C.sub.2 F).sub.n. Graphite fluoride possesses distinctive properties including unusually low surface energy and has acquired importance as a widely applicable industrial material. For example, graphite fluoride is of use as lubricant, as water- and oil-repellent and also as an active material for cell electrodes.
Graphite fluoride is obtained by directly fluorinating a solid carbon material with fluorine gas usually diluted with an inactive gas. However, mainly for the following reasons, the gas-solid contact reaction to form a desired polycarbon fluoride is not easy to industrially carry out and must be carried out under deliberately chosen and strictly controlled conditions, which are considerably variable depending on the kind and physical form of the carbon material. The reaction between solid carbon and fluorine gas to form, for example, (CF).sub.n or (C.sub.2 F).sub.n is highly exothermic, and the formed polycarbon fluoride is liable to further react with fluorine gas to decompose into solid carbon and gaseous fluorocarbons such as CF.sub.4 and C.sub.2 F.sub.6. Such decomposition reaction is also exothermic. Besides, some side reactions are likely to take place between solid carbon and fluorine gas to form gaseous perfluorocarbons. As a matter of inconvenience, both the decomposition reaction and side reactions can proceed at temperatures near the temperature suitable for the intended reaction.
As to the starting material, a wide selection can be made from various forms of carbon such as natural or synthetic graphite, petroleum coke, pitch coke, carbon black, activated carbon and carbon fibers. In most cases coke or graphite is used by reason of relative ease of converting into graphite fluoride, and the fluorination reaction is carried out at 300.degree.-500.degree. C. Usually, graphite fluoride powders produced in this way are 1-50 .mu.m in mean particle size.
Recently it is expanding to utilize excellent lubricity or water- and oil-repellency of graphite fluoride in composite materials comprising plastics, aqueous liquid or organic liquid as a principal component. For such applications, dispersibility of graphite fluoride becomes a very important factor. Since dispersibility of a powdery material depends greatly on the particle size, there is a keen demand for ultrafine particles, i.e. submicron particles, of graphite fluoride.
A conceivable way to obtain very fine particles of graphite fluoride is reducing the particle size of graphite fluoride powder obtained by the conventional synthesis process with a pulverizing machine. However, by this method it is very difficult and almost impracticable to obtain submicron particles of graphite fluoride. Even though the pulverizing operation is combined with classification operations, the ultimate particle is about 1 .mu.m at best. Besides, this method entails considerable cost.
Another way is fluorinating a carbon material in the form of ultrafine particles. In this case consideration must be given to the fact that the particle size of the obtained graphite becomes more than twice the particle size of the starting carbon material by reason of intrusion of fluorine atoms between the carbon network layers. That is, the particle size of the starting material needs to be smaller than 0.5 .mu.m for obtaining submicron particles of graphite fluoride. Therefore, the starting carbon material is limited to carbon black. However, it is not easy to industrially produce graphite fluoride from carbon black primarily because ultrafine particles of carbon black exhibit very high activity with fluorine and readily undergo the aforementioned side reactions to form gaseous perfluorocarbons. Accordingly the fluorination operation has to be performed with a countermeasure against the obstructive side reactions even though productivity of the operation is inevitably sacrificed. For example, JP-A 58-167414 proposes diluting 100 parts by weight of carbon black to be fluorinated with more than 50 parts by weight of graphite fluoride powder.
However, experiments have revealed that graphite fluoride carefully produced from carbon black does not greatly differ from ordinary graphite fluoride produced from petroleum coke in respect of dispersibility in water containing a surfactant or organic liquids such as alcohols and oils. Furthermore, even graphite fluoride produced from carbon black has a mean particle size larger than 1 .mu.m when measured by a sedimentation method using correlation of particle size with settling velocity of particles well dispersed in a liquid.
In JP-A 61-218697, we have shown that graphite fluoride excellent in lubricity and improved in dispersibility is obtained by using, as the starting material, a graphitized carbon black having in its crystalline structure interlayer spacings of 3.38-3.55 .ANG. determined by the X-ray diffraction (002). However, the particles of this graphite fluoride are not submicron when measured by a sedimentation method.